1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus that includes developers of the same hue and different densities, a control method thereof, and a storage medium, and more particularly to density control of an image forming apparatus.
2. Description of the Related Art
Accompanying the recent advances in image forming apparatuses, the level of needs relating to image quality has also increased. Electrophotographic image forming apparatuses have been proposed in which the number of colors is increased in comparison to the conventional image forming apparatuses that use toners (developers) of four colors, and some of those electrophotographic image forming apparatuses have been put into practical use.
For example, image forming apparatuses exist which add the toner colors of red, blue and green or gold, silver and fluorescent colors to the four conventional toner colors of cyan, magenta, yellow and black. Further, in the inkjet system, image forming apparatuses exist which add the toner colors of pale cyan and pale magenta to the four toner colors of cyan, magenta, yellow and black. In this specification, normal cyan and normal magenta are referred to as “dark cyan” and “dark magenta”, and pale cyan and pale magenta are referred to as “light cyan” and “light magenta”. The purpose of adding developers in this manner is to improve the respective image qualities. For example, an image forming apparatus in which toner of pale colors such as light cyan and light magenta are additionally provided, generally has a purpose of decreasing granularity, and is used as an image forming apparatus that is capable of achieving photograph-like high image quality.
Meanwhile, in image forming apparatuses according to the electrophotographic method, various controls are performed to enable stable image output with respect to fluctuations in the characteristics of image forming parameters (for example, developing bias) due to the influence of environmental conditions and the like. Further, in order to suppress increases in the image forming time due to such controls, attempts are made to realize effective control for stabilizing an image in a short time. For example, Japanese Patent Laid-Open No. 2003-228201 (hereinafter referred as JPA 2003-228201) discloses an effective image stabilizing control method that controls all tones using a single image for measurement.
In many cases of these image stabilizing controls, control is performed so as to most stably reproduce variations in sensitive range with respect to the visual sensitivity of humans. With respect to density, since humans are more liable to react to image quality or tints of slightly light portions of halftones than to those of dark portions, in many cases control is performed that places importance on those regions.
However, in a six-color image forming apparatus that uses toners for light cyan and light magenta (light toners) as well as black, yellow, normal cyan (dark toner), and normal magenta (dark toner), the relation between density on which importance should be placed and amount of applied toner changes with respect to dark toner and light toner.
Further, with regard to cyan and magenta, since a tone of a single color is reproduced by combining dark toner and light toner, the characteristics of a high density portion of light toner and a low density portion of dark toner are important. The problem thus arises that it is impossible to control all tones using a single image for measurement as described in JPA 2003-228201. More specifically, because of reproducing a tone of a single color by combining dark toner and light toner, when the stability of reproduction of a high density portion of light toner or a low density portion of dark toner is impaired, problems arise with respect to image quality, such as the occurrence of pseudo-contours.